1. Field of the Invention
The present invention is directed to acoustic loudspeakers including transducers having at least one stator plate closely adjacent to which is mounted a flexible diaphragm carrying an electrical circuit wherein magnetic elements carried by the stator plate cooperate with the electrical circuit of the diaphragm to drive the diaphragm when energy is applied to the electrical circuit. More particularly, the invention is directed to controlling the modal behavior of the flexible diaphragm by creating selective air resistance adjacent to the diaphragm in order to enable the diaphragm to function in a piston-like manner over a wide frequency range to thereby improve acoustic coupling and increase the acoustic output for the transducer of the loudspeaker.
2. Description of the Related Art
The mechanical properties of thin film or non-rigid diaphragm loudspeakers are such that, at high frequencies, air mass controls the high frequency operation and the mechanical stiffness of the diaphragms controls the lowest frequency performance of the loudspeakers.
Traditional low frequency planar transducer systems with stretched diaphragms utilize cloth, baffles or other forms of resistive loading to damp peaks in a system""s response. These resistive methods serve to decrease the output at resonance but do not maintain the output where the diaphragm exhibits out-of-phase modal problems at certain frequencies. The result is a system that exhibits poor frequency and phase response with lower overall acoustic output, for a given input power.
The support structure for a stator or grid of a planar magnetic or electrostatic loudspeaker, or other type of flat panel loudspeaker, incorporates perforations, holes, slots or no interface at all to allow the displacement of a thin diaphragm to couple acoustically to the surrounding medium, usually air. The diaphragm is frequently clamped at its edges and under tension.
It is generally assumed that the diaphragm behaves as a piston in operation, but a stretched membrane or non-rigid diaphragm will go through resonant modal patterns which create peaks and dips in a loudspeaker""s frequency response, even if the diaphragm is driven uniformly over its entire surface area. The peaks generally coincide with in-phase motion of the diaphragm and the dips result from portions of the diaphragm operating out-of-phase with other areas of the diaphragm. This results in a poor mechanical impedance match between the diaphragm and the surrounding air. Low frequency diaphragm modes occur independent of input power and result in low acoustic output at high input power when portions of the diaphragm are moving out-of-phase with other areas of the diaphragm.
Past methods for improving a system""s response have been equalization or diaphragms formed in cells where each cell has different resonance modes. U.S. Pat. No. 5,054,081 to West describes a form of diaphragm with segmented cells, each cell having a different first harmonic resonance frequency by virtue of area and tension. The sum of the resonance frequencies results in essentially flat frequency response. This method is suitable for very large diaphragms where there is sufficient area to break up the diaphragm into many small cells and recover otherwise unusable energy, but this method is not applicable for small transducers where the total area of the transducer occupies one cell. For small transducers there is insufficient area remaining to divide up and any division would result in cell resonance frequencies that are too high for wide range low frequency operation.
A significant problem for small planar magnetic and electrostatic transducers or loudspeakers is the ability to generate high acoustic output at low frequencies when compared to conventional moving coil transducers of similar size. Part of the problem is due to limited peak to peak displacement of the diaphragm in flat panel technologies. For planar magnetic loudspeakers, this is partially overcome with the use of modern magnetic materials to drive the diaphragm a primary limitation that remains is the high Q resonance which occurs at the fundamental resonance of the diaphragm. The high Q resonance occurs because of the elastic nature of the diaphragm material, the low stiffness of the diaphragm at one frequency, and the rigid support frame if used to support the diaphragm.
As magnetic circuits, magnetic materials and diaphragm materials improve, the peak to peak displacement capabilities of small planar woofers increases. This magnifies any low frequency resonance problems.
The compliance or elasticity of the materials which are desirable for use as diaphragms in planar magnetic or electrostatic transducers or loudspeakers are thin films such as Kapton(trademark) or Mylar(trademark) or various forms of vinyl chlorides such as Saran(trademark). All of these films exhibit a characteristic resonance frequency when they are under tension in a rigid frame as utilized in these types of loudspeakers. These characteristics result in a xe2x80x9cdrum likexe2x80x9d resonance which typically defines the lower limit of the frequency range of the transducers. Below this resonance frequency the stiffness of the diaphragm material is high and this limits output significantly. Use of a more compliant material or low tension causes loss of diaphragm control and high distortion from unstable motion of the diaphragms.
These drum like resonance""s cause limitations in the maximum obtainable output of a planar speaker at low frequencies because the rise in amplitude at these frequencies is typically 10-15dB or more. When a music signal is passed through a speaker, if a wide range signal in the music falls on the resonant frequency, the output rises substantially, beyond what is desirable and causes diaphragm bottoming against a stator component before the maximum usable sound output of the speaker has been reached at other frequencies. If this rise in resonance is reduced then the overall low frequency output of the speaker with music signals will increase.
At resonance, a speaker""s transfer efficiency is high, because of the very low stiffness of the diaphragm, but it is only at one frequency which is not useful in a loudspeaker intended for music reproduction.
Above the fundamental resonance, the diaphragm exhibits modal behavior where areas of the diaphragm move in different directions at the same time, even when driven uniformly over the entire surface of the diaphragm. The sum of the motions in different directions reduce the output. In some cases the reduction can be as much as 20 dB in acoustic output. The modal behavior and resonance modes are a significant problem with regards to achieving smooth low frequency response, high efficiency, and high acoustic output out of a small planar diaphragm.
FIG. 1 is a graphic which shows how a diaphragm clamped on two ends will respond to vibration inputs. Even with a sinusoidal input multiple harmonics are likely to exist on the diaphragm because of tension, peripheral termination and the velocity of sound along the diaphragm.
The desired motion is that of the first harmonic but the superposition of higher vibration modes such as the second harmonic when added to the fundamental result in diaphragm with less effective area and lower acoustic output.
High electrical impedance diaphragms in planar magnetic transducers or loudspeakers offer better control of modal behavior through increased back EMF, but are less efficient than low impedance diaphragms and the efficiency limitation prevents their use. Low impedance diaphragms are more desirable because of increased efficiency, however, they exhibit increased modal behavior problems. The present invention permits the use of low impedance diaphragms with greater control over the low frequency modal patterns.
U.S. Pat. No. 4,156,801 to Whelan describes a centrally driven diaphragm with baffles covering 80-85% of the non-driven area for the purpose of damping out-of-phase midrange resonance modes which cause frequency response irregularities. The design is optimized for operation from 255 hertz up and operation below this frequency is not discussed. Additional conductor area under the baffles has a less beneficial effect on the output of the speaker due to the very high stiffness of the air under the baffles.
U.S. Pat. No. 3,832,499 to Heil describes a magnetic circuit with a pole piece arrangement where the velocity of air between the pole pieces is accelerated but the effect is said to be beneficial only at very high frequencies.
As previously noted, current state of the art planar transducers utilize a uniform air load by means of perforations around the diaphragm or cloth to damp the resonance modes that result from tension. Resistive loading is accomplished by limiting the xe2x80x9copen areaxe2x80x9d exposed to the surrounding air by means of baffles or the application of cloth, foam or other materials in close proximity to the diaphragm. This reduces a rise in resonance, but at the expense of efficiency because resistive forms of damping waste otherwise useable energy at low frequencies. These forms of resistive loading also have little or no effect on the low frequency modal behavior of a diaphragm. Equalization is then required to make up for the effects of lost output. This increases the power requirement of an amplifier used to achieve enough output and decreases the power handling or thermal capacity of the loudspeaker.
The present invention is directed to acoustic loudspeakers which include transducers having a planar diaphragm mounted in a plane which is closely adjacent to at least one, and preferably two, stator plates on which are mounted magnetic elements. In preferred embodiments of the invention, the magnetic elements are permanent magnets of a bar type. An electrical circuit is carried by the planar flexible diaphragm so as to be responsive to the magnetic elements when electrical energy is passed through the electrical circuit thereby causing the diaphragm to vibrate at varying frequencies depending upon the power input.
It is one of the primary objects of the present invention to control a modal behavior of the flexible diaphragm such that it moves in a piston-like motion over a wide frequency range of predetermined interest and particularly for a flat panel type woofer operating in a frequency of 20 Hz to 200 Hz. To control the modal behavior, the present invention provides perforate openings in either one stator plate, if a single stator plate is used, or one or both stator plates, if two stator plates are used, wherein the openings are not uniformly distributed or sized so as to selectively create air pressure regions adjacent to the diaphragm that resist diaphragm motions on portions of the diaphragm where parts of the diaphragm would otherwise move out-of-phase with other parts of the diaphragm at certain operating frequencies. In preferred embodiments, the total open space created by the openings in the one or more stator plates is generally 20% or less of the total active or open surface area of the diaphragm. The active surface area is that which is defined between any mounting frame which supports the diaphragm at its outer edges and which is subject to vibration during the operation of the transducer.
In the preferred embodiments, the non-uniform spacing of the perforated openings in the transducer plate are such that the openings are not uniformly distributed across the surface of the plate but may be of greater density in one area of the plate than another. In other embodiments, the openings may vary in size and configuration along the stator plate so as to achieve the same resistive pressure of air adjacent to the diaphragm in areas of the diaphragm were modal control is desired.
In some embodiments, the selective air resistance adjacent the diaphragm may be created by providing openings of a first pattern in a front stator plate associated with a transducer and a different pattern in a rear stator plate associated with the same transducer. Further, in some embodiments, the openings through the stator plate may be flared or tapered such that they widen from a center of a stator plate to an outer surface or inner surface of the stator plate in order to further reduce undesired noise created by sound passing through the stator plate.
It is another object of the invention to use air stiffness to control diaphragms associated with planar magnetic and other flat diaphragm transducers at low frequencies in order to improve low frequency output of loudspeakers, and especially woofers, incorporating such transducers.
It is also an object of the present invention to provide for an improved method of increasing output, smoothing low frequency response and controlling low frequency resonance modes by making a stretched diaphragm behave like a piston through a wider range of frequencies in planar magnetic and other flat diaphragm type transducers used in loudspeakers and especially in woofers.
It is also an object of the invention to improve transfer efficiency of a planar magnetic or other flat type diaphragm transducer over a greater frequency range than is possible with current state of the art technologies.
It is another object of the invention to provide planar magnetic and other flat diaphragm transducers with one or more stator plates which are perforated non-uniformly so as to prevent interference of the stator plates with a vibrating diaphragm during operation of the transducers when the diaphragms are mounted closely adjacent to such stator plates.
Where the invention is used as a dipole, radiating sound from both sides, it is also an object of the invention to improve acoustic output by creating a pressure to velocity to sound pressure conversion beyond the boundary of the transducer such that the acoustic surface area is larger than the physical surface area of the transducer, lowering the dipole cutoff frequency.